The present invention relates to piezoelectric actuators, and more particularly to a piezoelectric latching actuator.
Electromagnetic actuators have been extensively used as relays due to their high switching performance and high voltage performance as compared with semiconductor counterparts. While electromagnetic relays have met with wide reception in various industrial applications, they still have a number of disadvantages in power consumption, heat loss and size due to the employment of excitation coil. Furthermore, magnetic flux generated by the coil tends to interfere with the operation of adjacent circuits.
Attempts have been made in recent years to utilize piezoelectric elements as a means for generating a driving force instead of the excitation coil. Piezoelectric elements are constructed of ceramic laminates of piezoelectric material. Two types of such laminates are in use. One is a bimorph element using a transverse piezoelectric effect which generates a displacement in a direction perpendicular to the direction of application of electric field, so that it bends in accordance with the applied field strength. The other is a multilayer element using a longitudinal piezoelectric effect which produces a displacement in a direction parallel to the direction of application of electric field, so that it expands in accordance with the applied field strength.
U.S. Pat. No. 4,383,195 issued to H. H. Kolm et al. discloses a piezoelectric snap actuator having piezoelectric bending elements of the bimorph type which are held in a curved configuration and a bistable snap-action layer sandwiched between the bending elements. Although satisfactory in terms of the amount of transverse displacement, the Kolm et al actuator is not energy efficient due to the transverse piezoelectric effect and a slow response due to the stress given to the piezoelectric elements by the curved configuration. A further disadvantage is that the displacement characteristic of the bimorph type actuator tends to vary with time if voltage is applied continuously.
On the other hand, the longitudinal effect piezoelectric element has a drawback in that the amount of its longitudinal displacement is much smaller than that obtained with the bimorph type element. While this problem may be solved by increasing the operating voltage, it exceeds the voltage limits of the associated actuator control circuitry. U.S. Pat. No. 4,454,442 issued to M. B. Hosking discloses a piezoelectric relay comprising a longitudinally expandable piezoelectric body and a single resilient elongate member of a dielectric material or a mechanical amplification means to amplify a minute dimensional change, typically a few micrometers, generated in the piezoelectric body for operating a relay contact. However, because of length limitations on the resilient member it is impossible to cause a relay contact to traverse between widely separated contacts.